1. Field of the Invention
The present invention relates generally to a running behavior control of a vehicle such as an automobile, and more particularly, to a stability control device of a vehicle for relieving drive wheels thereof from side slipping.
2. Description of the Prior Art
It is known in the art of running stability control of vehicles that, when a vehicle driven at the rear wheels runs along a curved course at high speed with engine braking, the vehicle is liable to spin due to a saturation of the tire grip of the rear wheels to the road surface against the centrifugal side force, while when a vehicle driven at the front wheels runs along a curved course at high speed with engine braking, the vehicle is liable to drift out due to a saturation of the tire grip of the front wheels to the road surface against the centrifugal side force, because the lateral grip limit of the tire decreases along with increase of longitudinal slip ratio of the tire due to the limit by the so-called friction circle whose radius corresponds to a vector addition of longitudinal grip force and lateral grip force of the tire. In order to meet with such a problem, it has been proposed in Japanese Patent Laid-open Publication 64-87844 filed by the same assignee as the present application to control the fuel supply system of the engine of a vehicle such that, when a more than certain limited slippage of the driving wheels is detected during an engine braked running of the vehicle, fuel cut is dissolved. According to this prior art, if the engine rotation speed increases beyond a predetermined threshold value when a vehicle is running with its intake throttle valve being fully closed, its fuel supply system is positively cut for the purposes of increasing the effect of the engine braking and reducing the emission of fuel components into the atmosphere. The dissolving of the fuel cut is to dissolve such a positive cutting of the fuel supply system when the vehicle is liable to slip due to a hard engine braking.
In order to further develop such a concept of attenuating the engine braking of a vehicle to retain the vehicle from getting instable by the driving wheels slipping against the road surface due to a hard engine braking, in copending patent application (1150) by the same inventor as in the preset application there has been proposed a device for controlling intake throttle of an engine of a vehicle, said device having a means for estimating an instability quantity representative of turn instability of the vehicle, a means for estimating lateral acceleration of the vehicle, a means for estimating friction coefficient of road surface, a means for estimating a brake limit of a pair of drive wheels based upon the lateral acceleration and the road surface friction coefficient, a means for estimating engine brake torque limit based upon the brake limit of the pair of drive wheels of the vehicle and the instability quantity, a means for estimating a target intake throttle value corresponding to the engine brake torque limit, and a means for attenuating intake throttling of the engine to the target intake throttle value when the intake throttling is tighter than the target intake throttle value, so that the attenuation of engine braking is continuously trimmed to be necessary minimum, so as thereby to avoid any abrupt change of engine braking effect, for further improvement of driving comfortability of the vehicle, while suppressing the total emission of fuel components into the atmosphere as integrated throughout the period of engine braked operation, the trimming of the engine braking attenuation being effected to trace a critical border between the stability and instability of the running behavior of the vehicle all though the driving period of the vehicle.
According to the above-mentioned copending patent application, the execution of the control by the device is restricted by the condition that the actual intake throttling is tighter than the target intake throttle value estimated based upon the instability quantity representative of turning instability of the vehicle, the lateral acceleration acting at the vehicle and the friction coefficient of road surface. In other words, if the actual intake throttling is not tighter than the target intake throttle value so estimated, the device of the invention can not be operated.